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                                <h1 id="basic-reducer-structure-and-state-shape">Basic Reducer Structure and State Shape</h1>
<h2 id="basic-reducer-structure">Basic Reducer Structure</h2>
<p>First and foremost, it&apos;s important to understand that your entire application really only has <strong>one single <a href="../../Glossary.html#reducer" class="glossary-term" title="">reducer</a> function</strong>: the function that you&apos;ve passed into <code>createStore</code> as the first argument.  That one single <a href="../../Glossary.html#reducer" class="glossary-term" title="">reducer</a> function ultimately needs to do several things:</p>
<ul>
<li>The first time the <a href="../../Glossary.html#reducer" class="glossary-term" title="">reducer</a> is called, the <code>state</code> value will be <code>undefined</code>.  The <a href="../../Glossary.html#reducer" class="glossary-term" title="">reducer</a> needs to handle this case by supplying a default <a href="../../Glossary.html#state" class="glossary-term" title="">state</a> value before handling the incoming <a href="../../Glossary.html#action" class="glossary-term" title="">action</a>.</li>
<li>It needs to look at the previous <a href="../../Glossary.html#state" class="glossary-term" title="">state</a> and the dispatched <a href="../../Glossary.html#action" class="glossary-term" title="">action</a>, and determine what kind of work needs to be done</li>
<li>Assuming actual changes need to occur, it needs to create new objects and arrays with the updated data and return those</li>
<li>If no changes are needed, it should return the existing <a href="../../Glossary.html#state" class="glossary-term" title="">state</a> as-is. </li>
</ul>
<p>The simplest possible approach to writing <a href="../../Glossary.html#reducer" class="glossary-term" title="">reducer</a> logic is to put everything into a single function declaration, like this:</p>
<pre><code class="lang-js"><span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">counter</span>(<span class="hljs-params">state, action</span>) </span>{
  <span class="hljs-keyword">if</span> (<span class="hljs-keyword">typeof</span> state === <span class="hljs-string">&apos;undefined&apos;</span>) {
    state = <span class="hljs-number">0</span>; <span class="hljs-comment">// If state is undefined, initialize it with a default value</span>
  }

  <span class="hljs-keyword">if</span> (action.type === <span class="hljs-string">&apos;INCREMENT&apos;</span>) {
    <span class="hljs-keyword">return</span> state + <span class="hljs-number">1</span>;
  } 
  <span class="hljs-keyword">else</span> <span class="hljs-keyword">if</span> (action.type === <span class="hljs-string">&apos;DECREMENT&apos;</span>) {
    <span class="hljs-keyword">return</span> state - <span class="hljs-number">1</span>;
  } 
  <span class="hljs-keyword">else</span> {
    <span class="hljs-keyword">return</span> state; <span class="hljs-comment">// In case an action is passed in we don&apos;t understand</span>
  }
}
</code></pre>
<p>Notice that this simple function fulfills all the basic requirements.  It returns a default value if none exists, initializing the <a href="../../Glossary.html#store" class="glossary-term" title="">store</a>; it determines what sort of update needs to be done based on the type of the <a href="../../Glossary.html#action" class="glossary-term" title="">action</a>, and returns new values; and it returns the previous <a href="../../Glossary.html#state" class="glossary-term" title="">state</a> if no work needs to be done.  </p>
<p>There are some simple tweaks that can be made to this <a href="../../Glossary.html#reducer" class="glossary-term" title="">reducer</a>.  First, repeated <code>if</code>/<code>else</code> statements quickly grow tiresome, so it&apos;s very common to use <code>switch</code> statements instead.  Second, we can use ES6&apos;s default parameter values to handle the initial &quot;no existing data&quot; case.  With those changes, the <a href="../../Glossary.html#reducer" class="glossary-term" title="">reducer</a> would look like:</p>
<pre><code class="lang-js"><span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">counter</span>(<span class="hljs-params">state = 0, action</span>) </span>{
  <span class="hljs-keyword">switch</span> (action.type) {
    <span class="hljs-keyword">case</span> <span class="hljs-string">&apos;INCREMENT&apos;</span>:
      <span class="hljs-keyword">return</span> state + <span class="hljs-number">1</span>;
    <span class="hljs-keyword">case</span> <span class="hljs-string">&apos;DECREMENT&apos;</span>:
      <span class="hljs-keyword">return</span> state - <span class="hljs-number">1</span>;
    <span class="hljs-keyword">default</span>:
      <span class="hljs-keyword">return</span> state;
  }
}
</code></pre>
<p>This is the basic structure that a typical Redux <a href="../../Glossary.html#reducer" class="glossary-term" title="">reducer</a> function uses.</p>
<h2 id="basic-state-shape">Basic State Shape</h2>
<p>Redux encourages you to think about your application in terms of the data you need to manage.  The data at any given point in time is the &quot;<em><a href="../../Glossary.html#state" class="glossary-term" title="">state</a></em>&quot; of your application, and the structure and organization of that <a href="../../Glossary.html#state" class="glossary-term" title="">state</a> is typically referred to as its &quot;<em>shape</em>&quot;.  The shape of your <a href="../../Glossary.html#state" class="glossary-term" title="">state</a> plays a major role in how you structure your <a href="../../Glossary.html#reducer" class="glossary-term" title="">reducer</a> logic.</p>
<p>A Redux <a href="../../Glossary.html#state" class="glossary-term" title="">state</a> usually has a plain Javascript object as the top of the <a href="../../Glossary.html#state" class="glossary-term" title="">state</a> tree. (It is certainly possible to have another type of data instead, such as a single number, an array, or a specialized data structure, but most libraries assume that the top-level value is a plain object.)  The most common way to organize data within that top-level object is to further divide data into sub-trees, where each top-level key represents some &quot;domain&quot; or &quot;slice&quot; of related data.  For example, a basic Todo app&apos;s <a href="../../Glossary.html#state" class="glossary-term" title="">state</a> might look like: </p>
<pre><code class="lang-js">{
  visibilityFilter: <span class="hljs-string">&apos;SHOW_ALL&apos;</span>,
  todos: [
    {
      text: <span class="hljs-string">&apos;Consider using Redux&apos;</span>,
      completed: <span class="hljs-literal">true</span>,
    },
    {
      text: <span class="hljs-string">&apos;Keep all state in a single tree&apos;</span>,
      completed: <span class="hljs-literal">false</span>
    }
  ]
}
</code></pre>
<p>In this example, <code>todos</code> and <code>visibilityFilter</code> are both top-level keys in the <a href="../../Glossary.html#state" class="glossary-term" title="">state</a>, and each represents a &quot;slice&quot; of data for some particular concept.</p>
<p>Most applications deal with multiple types of data, which can be broadly divided into three categories:</p>
<ul>
<li><em>Domain data</em>: data that the application needs to show, use, or modify (such as &quot;all of the Todos retrieved from the server&quot;)</li>
<li><em>App <a href="../../Glossary.html#state" class="glossary-term" title="">state</a></em>: data that is specific to the application&apos;s behavior (such as &quot;Todo #5 is currently selected&quot;, or &quot;there is a request in progress to fetch Todos&quot;)</li>
<li><em>UI <a href="../../Glossary.html#state" class="glossary-term" title="">state</a></em>: data that represents how the UI is currently displayed (such as &quot;The EditTodo modal dialog is currently open&quot;)</li>
</ul>
<p>Because the <a href="../../Glossary.html#store" class="glossary-term" title="">store</a> represents the core of your application, you should <strong>define your <a href="../../Glossary.html#state" class="glossary-term" title="">state</a> shape in terms of your domain data and app <a href="../../Glossary.html#state" class="glossary-term" title="">state</a>, not your UI component tree</strong>.  As an example, a shape of <code>state.leftPane.todoList.todos</code> would be a bad idea, because the idea of &quot;todos&quot; is central to the whole application, not just a single part of the UI.  The <code>todos</code> slice should be at the top of the <a href="../../Glossary.html#state" class="glossary-term" title="">state</a> tree instead.</p>
<p>There will <em>rarely</em> be a 1-to-1 correspondence between your UI tree and your <a href="../../Glossary.html#state" class="glossary-term" title="">state</a> shape.  The exception to that might be if you are explicitly tracking various aspects of UI data in your Redux <a href="../../Glossary.html#store" class="glossary-term" title="">store</a> as well, but even then the shape of the UI data and the shape of the domain data would likely be different.</p>
<p>A typical app&apos;s <a href="../../Glossary.html#state" class="glossary-term" title="">state</a> shape might look roughly like:</p>
<pre><code class="lang-js">{
    domainData1 : {},
    domainData2 : {},
    appState1 : {},
    appState2 : {},
    ui : {
        uiState1 : {},
        uiState2 : {},
    }
}
</code></pre>

                                
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